Electric lamps known as double-enveloped lamps include a light-source capsule, such as an arc tube, and an outer envelope surrounding the light source capsule. In such double enveloped lamps, there is a small probability that the light source capsule will burst. When such an event occurs, hot fragments of glass, or shards, and other capsule parts emanating from the burst capsule are forcibly propelled against the outer envelope. If the outer envelope also shatters, there is a potential safety hazard to persons or property in the immediate surroundings. Failure of the outer envelope is known as a "containment failure".
One way to avoid the safety hazard of containment failure is to operate the lamp within a protective fixture that is capable of containing such a failure. However, a protective fixture usually incurs additional cost, particularly if an existing fixture must be modified or replaced. Furthermore, a protective fixture reduces the light output of the lamp, and it may be more difficult and expensive to replace a lamp in a protective fixture.
A preferred solution to the containment failure problem is a lamp capable of self-containment. One known technique is to make the outer envelope stronger so that it contains the shattered light-source capsule. An outer envelope having a thick outer wall, in combination with a light-source capsule with a thin inner wall is disclosed in U.S. Pat. No. 4,598,225 issued July 1, 1986 to Gagnon. Another prior art technique is to shield the outer envelope from the effects of a burst light-source capsule. In U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl, et al, a light-transmissive enclosure located within an outer envelope surrounds a light source capsule and shields the outer envelope. See also U.S. Pat. No. 4,281,274 issued July 28, 1981 to Bechard, et al. Still another technique for containment is to reinforce the outer envelope or the shield. In U.S. Pat. No. 4,721,876 issued Jan. 26, 1988 to White, et al, a light transmissive shield is reinforced by a wire mesh. Wire mesh reinforcement of a light-source capsule is disclosed in U.S. Pat. No. 4,625,140 issued Nov. 25, 1986 to Gagnon. Containment techniques are also disclosed in pending application Ser. No. 90,983 filed Aug. 28, 1987, now abandoned, and assigned to the assignee of the present application.
While the above-referenced containment techniques are highly effective for some lamp types and sizes, they may have disadvantages when applied to other lamp types and sizes. For example, the use of a thick-walled outer envelope is effective for relatively small lamps. However, lamps of greater than 400 watts having a thick-walled outer envelope are so heavy that there is a possibility of the lamp falling out of the light fixture. Furthermore, thick-walled outer envelopes of large physical size are difficult to fabricate. While wire mesh reinforcement of a light transmissive shield is generally effective in achieving containment, the wire mesh absorbs an appreciable fraction of the output light from the light-source capsule. Furthermore, when the light-source capsule contains sodium, the proximity of a conductive wire mesh causes an effect known as sodium migration from the capsule and reduces the operating life of the lamp.
It is a general object of the present invention to provide improved double-enveloped lamps.
It is another object of the present invention to provide double enveloped lamps which can be safely operated without a protective fixture.
It is a further object of the present invention to provide double-enveloped lamps having an operating wattage greater than 400 watts wherein an outer envelope of standard thickness will contain a burst of the light source capsule.
It is still another object of the present invention to provide self-containing double enveloped lamps which have a high luminous output.
It is a further object of the present invention to provide double enveloped lamps wherein sodium migration is minimized.
It is yet another object of the present invention to provide double-enveloped lamps having a light transmissive shield reinforced with a nonconducting fiber mesh.
It is still another object of the present invention to provide double-enveloped lamps having a combination of the above features.